1. Technical Field
This invention relates to a light emitting device and a display apparatus using a plurality of light emitting devices, and in particular to an improved light emitting device and display apparatus which reduces light reflection at the surface to increase display contrast.
2. Description of Related Art
Today, light emitting diodes (LED's) capable of emitting each of the RGB colors (red, green, blue) and LED's capable of emitting white light at high luminance have been developed. As a result, the LED display apparatus, made up of an array of a plurality of LED's, is being applied in various fields. Compared with a light bulb, an LED has an extremely long lifetime, high efficiency, and furthermore is resistant to vibration. These types of properties are being put to good use, for example, in display applications such as advertisements, travel destination directions, and highway information. LED's are also being adopted for use as light sources for traffic lights; and for display screens from miniature to very large scale.
In particular, in an LED display capable of full color presentation, a single pixel is represented either by a close arrangement of LED's capable of emitting each of the RGB colors or by a single LED containing a plurality of light emitting diode chips capable of emitting each of the RGB colors. In either case, one pixel is represented by the mixture of light from each light emitting diode chip. An LED display is constructed by connecting together LED display apparatus, which are arrays of a plurality of LED's, in a body or frame such as a poly-carbonate resin carrier. Specifically, by fixing each LED display apparatus in an installation frame provided, for example, on the wall of a building, and by electrically connecting each LED display apparatus, a single LED display can be constructed.
LED's are primarily of the surface mount type with leads that mount on the surface of a substrate, or of the type with leads that pass through the substrate for attachment. Surface mount LED leads are bent to form a plane parallel to the substrate surface. LED's of the later type are formed with leads which are perpendicular to the substrate surface.
Of the LED's which have leads that can be surface mounted, chip-type LED's have a favorable reputation. A typical chip-type LED is formed by die-bonding an LED chip to the surface at the bottom of a cavity in a liquid crystal polymer package with leads formed in the package. Wire-bonding is performed as required. Next, package encapsulating resin is introduced into the cavity to at least cover the LED chip. On the other hand, a single type of encapsulating resin may be used to enclose and fix in place an LED chip and leads without using a package.
Of the LED's which are attached by passing leads through a substrate, lamp-type LED's have a favorable reputation. A typical lamp-type LED is formed by die-bonding an LED chip to the surface at the bottom of a lead cavity. Wire-bonding is performed as required. Next, encapsulating resin covering the LED chip is formed, for example, in the shape of a bullet.
In the present state of the technology, primarily LED's with leads which pass through a substrate, such as lamp-type LED's, can pass more current and emit with more luminance than chip-type LED's, which surface mount on a substrate. For reasons such as these, display apparatus with lamp-type LED's arrayed on a substrate are used primarily in outdoor applications requiring high luminance. In cases where surface mount chip-type LED's are used in a display apparatus, light-weight and thin-panel construction are advantages over apparatus with lamp-type LED's with leads which pass through a substrate. Further, since chip-type LED display apparatus can realize a narrower pitch between pixels and higher density arrays, fine resolution image display can be achieved. For these reasons, display apparatus with chip-type LED's surface mounted on a substrate are used primarily indoors where the distance from the display apparatus to the observation point is short.
Incidentally, inorganic or organic light scattering material is included inside the encapsulating resin of the LED's described above. By this construction, the efficiency of light extraction from the LED chip towards the observer is vastly improved.
However, accompanying the current expansion of fields of application for LED's and display apparatus using a plurality of LED's, as well as the demand for higher quality display, further improvement to reduce display contrast degradation has become necessary whether indoors or outdoors.
Specifically, when an LED is illuminated, external light such as sunlight or artificial lighting incident at certain angles reflects primarily off the surface of the LED encapsulating resin resulting in degradation in the display contrast. Light reflection also occurs at light emitting device side-walls thereby causing further display contrast degradation problems.
Even when an LED is not illuminated, external light can shine on the upper and side surfaces on the observer's side of the LED. As a result, light can reflect off at least LED surfaces producing glare which appears white to the observer.
Clearly, in the case of an LED display apparatus with a plurality of LED's arrayed on a substrate, display contrast degradation can also occur for the same reasons described above. Further, external light not only reflects off LED surfaces but also off the substrate surface to reduce display contrast even more. Even when LED's are not illuminated and even when the substrate itself is, for example, colored with a dark color, the display apparatus itself can appear white because of external light reflected off the substrate or LED surfaces towards the observer.
Display quality of an LED or LED display apparatus is maintained by an inherent dark coloration such as black or dark blue. However, as a result of light reflection at the surface, not only is display contrast reduced, but LED and LED display apparatus quality loss due to glare and white appearance is a problem.
Inside an LED, light scattering material of relatively small particle size, such as silica, is included inside the encapsulating resin covering the LED chip to improve light producing efficiency. However, due to the influence of factors such as encapsulating resin surface tension and specific gravity of the encapsulating resin and the light scattering material, the light scattering material ends up completely encased in encapsulating resin after final hardening, and the surface of the encapsulating resin ends up with a smooth surface. For this reason, even though the extraction efficiency of light produced inside the device can be improved, external light striking the smooth surface of the encapsulating resin produces glare. Here, the term “smooth surface” refers to an encapsulating resin surface, as described above, with roughness insufficient to scatter light from an external source. Namely, protrusions formed in the encapsulating resin surface due to an inner layer of different material such as light scattering material are not sufficient to scatter light from an external source.
The present invention was developed considering the problems described above. Thus, it is an object of the present invention to provide a light emitting device and a display apparatus using a plurality of light emitting devices which can drastically reduce display contrast loss due to light from external sources. The above and further objects and features of the invention will be more fully apparent from the following detailed description in conjunction with the accompanying drawings.